This invention relates to a patch antenna, and in particular to a wideband patch antenna fed by an L-shaped probe. The invention further relates to antenna arrays comprising a plurality of patch antennas.
Microstrip patch antennas have become very popular in recent years for a wide variety of applications. They have a number of advantages including low cost, small size and light weight that make them very suitable, for example, for use in Personal Communication Systems.
A conventional microstrip patch antenna comprises a patch of a given geometrical shape (eg circular, rectangular, triangular) spaced from a ground plane and separated from the ground plane by a dielectric. Normally the patch is fed by means of a coaxial feed.
One drawback, however, with microstrip patch antennas is that they have a relatively low bandwidth and are not therefore generally suitable for broad bandwidth applications. A number of approaches have been taken over the years to try and increase the bandwidth of microstrip patch antennas. Prior proposals, for example, have included adding second parasitic patch electromagnetically coupled to the driven patch (R. O. Lee, K. F. Lee, J. Bobinchak Electronics Letters Sep. 24, 1987 Vol.23 No.20 pp1017-1072), tuning out the probe inductance with a capacitive gap which allows the use of a thick substrate (P. S. Hall Electronic Letters May 21, 1987 Vol.23 No.11 pp606-607), and including a U-shaped slot in the patch antenna (K. F. Lee et al IEE Proc. Microw. Antennas Propag., Vol.144 No.5 October 1997).
None of these prior approaches to the problem are ideal however. The use of a parasitic patch overlying the driven patch undesirably increases the thickness of the antenna. The capacitive gap needs to be fabricated with high precision. Introducing a U-shaped slot gives an antenna with high cross-polarization and cannot be used for circularly polarized radiation. These remains a need for a simple patch antenna design that has increased bandwidth without introducing further drawbacks.
Another example of the prior art is shown in U.S. Pat. No. 4,724,443 (Nysen). Nysen describes a patch antenna in which a stripline feed element is coupled electromagnetically to a patch, and in which one end of the strip (which is parallel to the patch) is connected by the inner conductor of a coaxial cable (which is normal to the patch). In this design only the strip is coupled to the patch, and the antenna is not wide in its bandwidth.
According to the present invention there is provided an antenna comprising a patch disposed above a ground plane and spaced therefrom by a dielectric material, and an L-shaped probe disposed between said patch and said ground plane, said L-shaped probe having a first portion normal to said ground plane and said patch, and a second portion parallel to said ground plane and said patch, said antenna further comprising means for connecting said probe to means for transmitting a signal to or from said antenna, and said first portion of said L-shaped probe extending through said ground plane, wherein both said first portion and said second portion of said L-shaped probe are adapted to be electromagnetically coupled to said patch.
In a particularly preferred embodiment of the present invention the first portion of the L-shaped probe has an inductive reactance determined by the length of the first portion, and the second portion has a capacitive reactance determined by the length of the second portion, the respective lengths of the first and second portions being selected such that the inductive reactance of the first portion is cancelled by the capacitive reactance of the second portion.
The patch may be rectangular, circular or triangular or indeed any other geometric shape. The patch may also be provided with a slot which may be of various shapes, for example a U-shaped slot.
In one particularly preferred example of the invention there is provided an antenna comprising a rectangular patch of dimensions Wx by Wy disposed above a ground plane and spaced therefrom by a dielectric material (eg a foam material) by a distance H, and an L-shaped probe disposed between said patch and said ground plane, said L-shaped probe having a first portion of length Lh normal to said ground plane and said patch, and a second portion of length Lv parallel to said ground plane and said patch and spaced from one edge of the patch by a distance S, said antenna further comprising means for connecting said probe to means for transmitting a signal to or from said antenna, wherein said antenna has the dimensions:
Wx=0.45xcex
Wy=0.375xcex
H=0.099xcex
Lv=0.16xcex
Lh=0.083xcex
S=0.03xcex
The antenna may be a single antenna with one patch and one L-shaped probe. However, viewed from another aspect the invention provides an antenna array comprising a plurality of patches disposed above a ground plane and spaced therefrom by a dielectric material, each said patch having a respective L-shaped probe disposed between said patch and said ground plane, each said L-shaped probe having a first portion normal to said ground plane and said patch, and a second portion parallel to said ground plane and said patch said antenna array further comprising a transmission network connecting said probes to each other and to means for transmitting a signal to or from said antenna array.
Such an antenna array may take a number of forms. In its simplest form the array may comprise two patches with their respective L-shaped probes being connected by a single transmission line. More elaborate arrays may also be formed, for example a 4xc3x971 array with four L-shaped probes connected to a single transmission line. Or a 2xc3x972 array formed of two pairs of patches with the L-shaped probes of each patch being connected by a transmission line, and the two transmission lines being connected by a third so as to form a single transmission network to which all probes are attached. Two such 2xc3x972 arrays may then be connected together to form a 4xc3x972 array.
Another possibility is that in an antenna array the individual L-shaped probes may be connected by a microstrip feedline disposed below the ground plane.
A preferred particular example of an antenna array comprises two rectangular patches having dimensions Wxc3x97L disposed above a ground plane and spaced a distance H1 therefrom by a dielectric material (eg air), each said patch having a respective L-shaped probe disposed between said patch and said ground plane, each said L-shaped probe having a first portion normal to said ground plane and said patch, and a second portion parallel to said ground plane spaced by a distance H2 therefrom and having a length D, said antenna array further comprising a transmission line connecting said probes to each other and to means for transmitting a signal to or from said antenna array, said transmission line being parallel to said ground plane and spaced therefrom by a distance H3, said transmission line having a thickness S, a width S, and a length xcex/2, and wherein
W=0.42xcex
L=0.377xcex
H1=0.127xcex
H20.09xcex
H3=0.033xcex
S=0.039xcex
D=0.171xcex
It will also be understood that the patch antennas may be spaced from the ground plane by any form of dielectric material (including air) or indeed by multiple layers of differing dielectric materials.